Active filters



United States Patent ACTIVE FILTERS John M. Wozencraft, Washington,D.C., assignor to the United States of America as represented by theSecretary of the Army Application March 21, 1955, Serial No. 495,833 6Claims. (Cl. 250-27) (Granted under Title 35, U. S. Code (1952), sec.26.6)

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

The invention generally relates to a new and useful active filtercircuit. More specifically, the present invention is directed to anotherwise passive filter circuit in which the initial circuit conditionsare set at a predetermined time and the output is sampled at a laterpredetermined time whereby the filter operation has desired responsecharacteristics.

One type of use to which the present invention is adapted is thereception of a signal which is made up of a succession of C.W. pulses,any one pulse of which may or may not exist, the pulse repetition ratebeing constant and the carrier frequency of the pulse being specified.This type of signal occurs, for example, in the operation of radioTeletype systems, and the invention will be described in connection withsuch a system, although it is to be understood that it is not limited tosuch use.

By way of explanation and background, the operation of a simple passivefilter network consisting of a series resistor and a shunt parallelresonant network will be described. The filter elements are seriesconnected to an input source and have an output circuit connected acrossthe parallel resonant circuit. If this type of filter circuit has a C.W.pulse of alternating current applied to its input circuit the outputvoltage at the instant of application of the pulse will be substantiallyequal to zero. The output voltage, however, builds up with the passageof time and by a proper choice of constants this increase in outputvoltage can be made to bear, within limits, a substantially linearrelationship to the elapsed time from the application of the pulse. Atthe termination of the pulse the output voltage of the filter does notdrop immediately to zero. The voltage across the parallel resonantcircuit follows a decay characteristic which has a duration comparableto that which would have been required for the build-up of voltageacross it to go to completion.

In certain types of systems where the time factor becomes irnportant thepassage filter network described above possesses definite limitations.When it is desired, for example, to examine with a single narrow-bandfilter input C.W. pulses occurring one right after the other, the decaytime of the filter prevents itsoutput voltage from presenting a truepicture of the voltage resulting from the second pulse due to carryoverfrom the first pulse. This problem has been solved in prior art by usinga plurality of passive filter networks with a synchronous switchingsystem, whereby the input voltage during a first discrete time intervalis applied to a first passive filter network, the input voltage is thenswitched to a second passive filter network during the second discretetime interval, and so on. A rotating drum, or other equivalentarrangement, carrying a sufficient number of passive filter networksused in sequence, allowed sufficient time for the output voltage of eachfilter to decay before input voltage was again applied thereto. In thismanner a relatively true picture of voltage conditions over each timeinterval could be obtained.

The present invention utilizes only a single filterto obtain the sameresults derived from the multiple filters a fie and synchronousswitching system described above. The active filter disclosed is set toan initial circuit condition at the beginning of each discrete timeinterval. Thefilter then acts as a passive filter whose output voltagebuildsup in a linear manner with respect to time and is sampled at theend of that discrete time interval. The filter circuit is then reset tothe initial circuit condition at the beginning of the next followingtime interval.

It is a principal object of the present invention to provide a new anduseful active filter circuit capable of evaluating input voltages overdiscrete and immediately successive intervals of time.

It is a further object of the present invention to provide an activefilter circuit whose impulse response is matched to the input signal atcertain specified times, that is, provides the maximum signal to noiseratio when the signal is corrupted by white Gaussian noise.

It is a still further object of the invention to provide a filtercircuit which functions as an interval integrator to evaluate inputvoltages thereto over discrete and immediately successive intervals oftime.

Other objects, and many attendant advantages of the novel filter system,will become apparent as the same becomes better understood from thefollowing detailed description when considered in conjunction with theaccompanying drawings, wherein:

Fig. l is a schematic drawing, partially in block form,

of a system for the reception of pulsed C.W. signals which incorporatesa filter system constructed in accordance with the principles of theinvention, and

Figs. 2 through 6 are diagrams of voltage waveforms occurring atparticular locations in the circuit of Fig. 1,

which are used to explain the operation of the circuit.

Referring first to Fig. l of the drawings, a circuit is shown whichincorporates the novel filter system as a part thereof. This circuitcomprises a filter 11 having a pair of input terminals 13 and 15, a pairof output terminals 17 and 19 and a control input terminal 21. The inputterminal 15 is connected through a resistor 23 to one terminal of aparallel resonant circuit 24 and the output terminal 17. The otherterminal of the parallel resonant circuit 24 is connected to the inputterminal 15 and the output terminal 19.

The parallel resonant circuit is made up of a condenser 25 and aninductance 27. The circuit 24 is effectively shunted by a resistanceelement 29 which is shown in dotted form to indicate that it may or maynot have a physical existence. It may be representative of the resistivecomponents of the condenser and inductance elements of the circuit or itmay actually be a physical resistor. In still other cases where a veryselective circuit is desired, the element 29 may actually include a Qmultiplier such as the positive feedback, negative resistance circuits,known to the prior art. The requirements for element 29 of the circuit24 are determined by the nature of the input signal to the filter andwill be discussed more fully later.

The circuit 24 is further shunted by a circuit which includes a diode 31and a resistor 33 connected in series. The junction point between thecathode of the diode 31 and the resistor 33 is connected to one terminalof a art and may, for example, be of the type commonly used intelevision receivers. The synch separator or filter is connected to thefilter input terminals and passes only signals of a particular frequencyapplied thereto. Pulses of this frequency received at the input arepassedto'the automatic frequency and phase control unit where they arecompared with the output voltage generated by the local oscillator 45.Any shift in phase between the locally generated signal and the receivedpulses produces a control voltage in element 43 which is applied to afrequency control element such as a reactance tube contained within theoscillator 45. This circuit operates in a known manner to retainsynchronism and phase position of the locally generated and receivedsynchronizing pulses.

The output voltage of the oscillator 45 is fed to a wave shapinggenerator 49. The wave shaper and harmonic generator circuits operate ina known manner to convert the output voltage of the oscillator togenerally squared output trigger voltage of the shape shown in Fig. 3.The wave contains relatively narrow negative-going trigger pulseportions, a, accurately synchronized with the incoming synch pulses. Theoutput voltage of the wave shaping network is applied directly to thecoil 56 of a sampling device, in this case a biased relay 55, andthrough a time delay multivibrator circuit 51 to the control inputterminal 21 of the filter network. The delay multivibrator reproducesthe input pulse with a time delay small compared to the pulse duration.This time delay is illustrated at C in Fig. 5.

The voltage at the output terminals 17 and 19 of the filter network 11is applied to a detector circuit 53 which is illustrated as aconventional diode detector. The rectified output of the diode detector53 is applied at the end of each pulse through the contacts of thebiased sampling relay 55 to the input of a holding circuit 58, whoseoutput in turn is applied to the system output terminals 59 and 61. Theholding circuit 58 may be a bi-stable multivibrator circuit whichoperates in response to the sampled detector output voltage in a mannerwhich will be more fully described hereinafter.

The operation of the circuit of Fig. 1 will now be described. Attentionis directed to Fig. 2, which illustrates the waveform of a typicalvoltage input to the filter terminals 13 and 15. The instantaneousvoltage magnitude is plotted along a time axis. It will be noted thatthe voltage waveform is divided into discrete intervals of equal timeduration T, commencing at a time T and running through a time T Overcertain of the intervals of duration T, e.g., from T to T T to T T to Tand T to T the voltage is of the form:

e=E cos 21rft where,

E =the maximum voltage f=the frequency of the carrier voltage, andt=time Over other intervals of duration T, e.g., T to T and T to T thevoltage is of the form:

The filter operates to evaluate the voltage input in each of theintervals of duration T separately and distinctly from each otherinterval. The input to terminals 13 and 15 also includes synchronizingor synch pulses, of a frequency other than that of the carrier 1, whichare not illustrated in Fig. 2. These synch pulses, in a manner known tothe art, occur at definite time intervals with respect to the timeintervals T and may, for example, occur at the start of every sixth timeinterval. These pulses are taken from the input terminals through thesynch filter circuit 41 and are applied to the automatic frequency andphase control unit 43. The local oscillator 45 is free running andnormally generates one cycle of output in each period T. The phase andfrequency control unit 43 compares the phasing of the output pulses fromthe oscillator 45 with that of the incoming synch pulses from the filter41 and develops a control voltage which is applied to a control elementsuch as a reactance tube included in the oscillator 45 to control itsoutput frequency. As is well known in the art, even though a phasecomparison is made only at spaced intervals, the phasing of theoscillator output voltage can be accurately controlled in this manner.

The output voltage from the oscillator 45 is fed through the harmonicgenerators and wave shaping networks 49 to convert it to the waveformillustrated in Fig. 3. The output voltage of the harmonic generators andwave shaping networks has a relatively high positive D.C. level over thegreater portion of the time, with negative going pulses, a, occurring attimes T T etc., spaced by time intervals T. As may be seen by acomparison of Figs. 2 and 3 which are plotted to the same time scale,the negative pulses, a, occur at the start of each CW pulse interval.

Referring back to Fig. l, the output of the harmonic.

generator and wave shaping network 49 is applied by way of the delaymultivibrator and the gaseous discharge tube 35 to the cathode of theswitching diode 31. The

output voltage of the delay multivibrator maintains the same waveform asthat illustrated in Fig. 3, but is shifted along the time axis by anamount small compared to the pulse duration. When the output of thedelay multivibrator is at a high D.C. level the gaseous discharge tube35 is conducting and the cathode of diode 31 is maintained at arelatively high positive potential. The diode 31 cannot conduct underthese conditions. When, however, a negative-going pulse, a, such as thatat time T occurs the potential applied to the gaseous discharge tube 35falls below the critical or threshold level of the tube, shown at b (seeFig. 3), and the discharge tube ceases to conduct, becoming effectivelyan open circuit. The cathode of diode 31 is no longer maintained at itsrelatively high potential and the diode becomes conducting, connectingresistor 33 across the parallel resonant circuit in shunt with elfectiveresistance 29 of the circuit.

The resistor 33 is very small in comparison to the resistor 23, and whenit is connected by the diode any voltage existing across the parallelresonant circuit is rapidly discharged through the low resistance path.The filter output voltage is thus quickly reduced to zero. This actionis shown in Fig. 4. After the delayed negative-going pulse, a, the diodereturns to its non-conducting state and any voltage of the frequency f,for which the filter is designed, subsequently applied to the filtercauses the voltage across the parallel resonant circuit 24 to build upwith respect to time in the manner shown in Fig. 4. At the time T thenegative-going pulse, a, occurs to again reduce the filter outputvoltage to zero and the filter thereafter responds to any input voltageof frequency f applied thereto in the next succeeding pulse interval T.The filter system is reset at the start of each interval.

In order that the envelope of filter output during each pulse besubstantially equal to the integral of the envelope of the voltageinput, the bandwidth of the resonant circuit must be very much less thanthe reciprocal of the pulse duration T. For example, for a pulseduration of 20 milliseconds, a bandwidth less than 10 cycles issufiiciently small. The exact value of bandwidth is not critical.

The voltage output of the filter is applied to the detector circuit 53where it is rectified to produce the detector output waveformillustrated in Fig. 5 of the drawing. The carrier voltage is eliminated,the positive D.C. envelope voltage only remaining.

The output of the harmonic generators and wave shaping network 49 isalso applied directly to a sampling device illustrated in Fig. 1 as abiased relay. The biased relay includes an energizing coil 56 and a biascoil 57 connected to a source of voltage (not shown). So long as thevoltage output of element 49 is maintained at its high positive D.C.level the flux of the coil 56 cancels the flux of the bias coil 57 andthe contacts of relay 55 remain open. When a negative-going pulse, a,occurs the flux of the bias coil 57 is no longer cancelled and thearmature is actuated by the fiux of the bias coil to close itsassociated contacts and connect the output of the detector 53 to theinput of the holding circuit 58 for the duration of the pulse, a. Sincethe voltage of the harmonic generators and wave shaping networks 49 isapplied directly to the coil 56 of the sampling relay 55 without passingthrough the delay multivibrator 51, it will be apparent that thecontacts of the sampling relay are closed just prior to the resetting ofthe filter output voltage to zero. In other words, the output voltage ofthe filter is sampled at the end of one pulse interval and the filter isreset at the beginning of the next pulse interval.

The detector output voltage sampled in the manner described above isapplied as the input to the holding circuit 58. The holding circuit maybe, as previously mentioned, a bi-stable multivibrator circuitdeveloping an output voltage determined by a pulsed input thereto andholding such an output voltage until the condition of the circuit isaltered by a succeeding pulsed input. This action is illustrated in Fig.6. The filter output is sampled at a time just prior to T At this timethe filter and detector outputs are at a high level due to the fact thata C.W. input voltage existed between times T and T The sampled detectoroutput applies a positive pulse to the holding circuit, tripping it toproduce a positive output voltage at the instant of sampling. Thisoutput voltage is held by the holding circuit until a time just prior toT when the detector output is again sampled. The detector output is atzero level at this time and this output applied to the holding circuittrips it back again to reduce its positive output voltage. The outputvoltages of the holding circuit thus persist for intervals equal to T,but are shifted along the time axis due to the fact that they aredependent upon sampling which occurs at the termination of the C.W.pulse intervals. The resultant output voltage of the holding circuit isshown in dotted lines with a small square at the head of each lineindicating the time of occurrence of the sampling pulse initiating theparticular output level.

It will be apparent that each pulse level in the output wave of Fig. 6is dependent upon, and is determined, by the integral of the envelope ofthe C.W. input voltage to the filter during a corresponding timeinterval. The single active filter circuit used responds only to inputvoltages of the proper frequency occurring within this particular timeinterval, the periodic resetting action of the filter eliminating theefiect of any voltages outside this interval.

While the novel filter system has been disclosed in connection with aparticular form of pulsed C.W. pulse input thereto, it is clear that theinvention is not limited to such systems. For example, with aninputcomposed of discrete pulses of direct current the parallel resonantcircuit could be replaced by a condenser and the timing wave for theresetting and sampling circuits generated in any desired fashion so longas they are properly coordinated with the intervals between successivepulse intervals.

Furthermore, while there has been described what are at present deemedto be the preferred embodiments of the invention, it will be apparent tothose skilled in the art that various changes and modifications may bemade therein without departing from the invention, and it is thereforethe aim of the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In an active filter system adapted to respond to a voltage which isgenerated in successive discrete intervals of time with a randomprobability of the existence of voltage in any particular time interval,the combination comprising a pair of filter input terminals, a seriesimpedance and a shunt impedance connected in series across said inputterminals, a pair of filter output terminals connected across said shuntimpedance, switching means actuated in synchronism with the start ofeach discrete time interval to momentarily shunt said output terminalsand reduce the filter output voltage to zero, and sampling meanssynchronously actuated at the termination of each discrete time intervalto momentarily connect the said filter output terminals to a utilizationdevice.

2. In an active filter system adapted to respond to a voltage which isgenerated in successive discrete intervals of time with a randomprobability of the existence of voltage in any particular time interval,the combination comprising a pair of filter input terminals, a seriesimpedance and a shunt impedance connected in series across said inputterminals, a pair of filter output terminals connected across said shuntimpedance, a source of trigger voltage synchronized with said discreteintervals of time, synchronous switching means responsive to saidtrigger voltage to momentarily shunt said filter output terminals at thestart of each time interval, and a second switching means responsive tosaid trigger voltage to momentarily connect the output terminals of saidfilter to a utilization device at the termination of each discreteinterval of time.

3. In an active filter system adapted to respond to an alternatingvoltage having a particular frequency which is generated in successivediscrete intervals of time with a random probability of the existence ofvoltage in any particular time interval, the combination comprising apair of filter input terminals, a series impedance and a shunt impedanceconnected in series across said input terminals, a pair of outputterminals connected across said shunt impedance, a source of triggervoltage synchronized with said discrete intervals of time, synchronousswitching means responsive to said trigger voltage to momentarily shuntsaid filter output terminals at the start of each time interval, andsecond switching means responsive to said trigger voltage to momentarilyconnect the output terminals of said filter to a utilization device atthe termination of each discrete interval of time.

4. An active filter system according to claim 3 wherein said shuntimpedance is a parallel resonant circuit tuned to the frequency of saidalternating voltage.

5. An active filter system comprising a normally passive filter networkhaving input and output terminals, means adapted to connect said inputterminals to a source of voltage which is generated in successivediscrete intervals of time with a random probability of the existence ofvoltage in any particular time interval, switching means actuated insynchronism with the start of each discrete time interval to momentarilyshunt said output terminals and reduce the filter output voltage tozero, and sampling means synchronously actuated at the termination ofeach discrete time interval to momentarily connect the said outputterminals to a utilization device.

6. An active filter system comprising a normally passive filter networkhaving input and output terminals, means adapted to connect said inputterminals to a source of voltage which is generated in equal successivediscrete intervals of time with a random probability of the existence ofvoltage in any particular time interval, a source of trigger voltagesynchronized with said discrete intervals of time, synchronous switchingmeans responsive to said trigger voltage to momentarily shunt saidoutput terminals at the start of each time interval, and a secondswitching means responsive to said trigger voltage to momentarilyconnect the said output terminals to a utilization device at thetermination of each discrete time interval.

References Cited in the file of this patent UNITED STATES PATENTS2,157,312 Wright May 9, 1939 2,258,877 Barber Oct. 14, 1941 2,293,135Hallmark Aug. 18, 1942 2,416,308 Grieg Feb. 25, 1947 2,500,536 GoldbergMar. 14, 1950 2,532,338 Schlesinger Dec. 5, 1950

